Recent decades have seen a continued depletion of high-grade mineral resources and, concomitantly, a growing demand for precious metals. The demand for gold is unbowed. At the same time, the awareness for environmental problems associated with conventional mining techniques has grown significantly.
Gold (Au):
Gold is one of the rarest elements on earth. In seawater, which constitutes the largest reservoir of gold, its concentration is only 0.01 mg/m3, while on average 1-2 g/t is found in the upper crust of earth. In this environment, gold mostly occurs as pure metal (Au0), electrum (Ag/Au), gold-containing minerals and tiny inclusions are found in large volumes of material, usually rock. Furthermore, it is found (often in association with quartz) as telluride (AuTe2) and selenide (AuSe2) or locked in the lattice of minerals such as pyrite and arsenopyrite (invisible gold). Yields of gold obtained by commercial mining are currently between 0.5 and 13.7 g gold/t rock, with a tendency to increasingly exploit low-grade ores due to a shortage in higher grade ones.
Silver (Ag):
Silver is about 20 times more abundant than gold. The majority of silver commercially accessible to date is deposited as metallic silver. But also sulphidic minerals (Ag2S, acanthite) and AgCl (cerargyrit) often occur. Like gold, also silver minerals are often found embedded in silica matrices (quartz) in particle sizes in the range of nano- to micrometers.
A number of living microorganisms, but also nonviable, inactivated cells have the ability to bind metal ions. In the first case, metal binding can occur via adsorption to the cell surface or via active intracellular accumulation of metal ions. In the latter case of nonviable, inactivated cells—that is often referred to as biosorption—metal ion binding is believed to occur exclusively via surface adsorption. The biosorption capacity as a general characteristic of biomass results from the presence of chelating groups (e.g. carboxyl-, amide-, hydroxyl-, phosphate-, and thiol-groups) contributed by carbohydrates, lipids and proteins that are displayed on the cell surface. It has been described that amounts of metals of up to 50% of the cell dry weight can be accumulated by biomass (Vieira and Volesky (2000) “Biosorption: a solution to pollution?” Int Microbiol 3(1): 17-24). U.S. Pat. No. 5,055,402 discloses a process for removing metal ions from aqueous solution, using a matrix prepared from metal-binding microorganisms that have been immobilized and heat-inactivated at temperatures of 300-500° C. EP 0 432 935 B1 describes the adsorption of soluble metal-cyanide complexes also from aqueous solution by living biomass.
Traditionally, precious metals such as gold and/or silver have been recovered by placer (sediment) mining or hard rock mining using gravity and pyrometallurgical methods. Due to the exhaustion of metal-rich ores, hydrometallurgical techniques are increasingly employed to recover precious metals from low-grade sources. Methods for precious metal recovery, particularly gold, are work-intensive and require the use of heavy machines as well as of hazardous and recalcitrant chemicals. Nowadays, about 90% of the common industrial processes for the recovery of precious metals are based on cyanidation methods, since cyanide is one of the very few substances that are able to dissolve gold. In order to allow cyanide ions or other compounds to access a large portion of the metal enclosed in its ores, ores are generally ground to small particle sizes. However, the conventional method of separation of precious metals using cyanide leaching is problematic for the environment as well as for human health. Therefore, more environment-friendly processes would be desirable.
WO2009/130006 describes a procedure for isolating metals, notably precious metals, or their compounds from particulate material such as mineral ores using certain biomass. The biomass binds the metal or the metal compound by cell components of the organisms. After separation of the biomass from unbound material of the particulate material, the metal or a compound of said metal can be isolated from the biomass. However, identifying biomass or other material suitable for such process is not an easy task.
Departing from the prior art, it is an object of the invention to provide a methodology for identifying material capable of binding a heavy metal such as gold and/or silver. It is another object of the invention to provide a process of isolating a heavy metal such as gold and/or silver from material containing the heavy metal such as gold and/or silver.